A spark plug used for an internal combustion engine or a similar component, for example, includes an insulator, a center electrode, a tubular metallic shell and a ground electrode. The insulator has an axial hole extending in the axis direction. The center electrode is disposed to be inserted into a front end side of an axial hole. The metallic shell is disposed at the outer circumference of the insulator. The ground electrode is secured to the front end portion of the metallic shell.
One known type of a spark plug (a so-called parallel electrode type and a transverse discharge type) has a gap between the front end portion of the ground electrode and the front end portion of the center electrode. Applying a voltage to the gap generates discharge in the air. In another known type of a spark plug (a so-called surface discharge type), the front end surface of the ground electrode is disposed so as to face the outer peripheral surface at the front end side of the center electrode. A part of a discharge path becomes a path that goes along the surface of the insulator. Nowadays, a plasma jet ignition plug has been proposed. In this plasma jet spark plug, the front end of the center electrode is positioned at a rear end side with respect to a front end of an insulator. Plasma is generated by turning on electric power between both electrodes where discharge is generated.
Generally, to obtain excellent thermal resistance, dielectric strength characteristics and mechanical strength, the insulator is formed with an alumina-based sintered body. The alumina-based sintered body is obtained by sintering an insulating material with a main constituent of alumina (Al2O3). Furthermore, in formation of the insulator, to achieve reduction in sintering temperature and improvement of sinterability (denseness of the insulator), for example, silicon oxide (SiO2), calcium oxide (CaO), or magnesium oxide (MgO) is employed as sintering additive.
Additionally, to ensure excellent dielectric strength characteristics even under high temperatures, a technique that contains a rare earth element (for example, scandium (Sc), Y (yttrium), or a lanthanum (La)) in an insulator is proposed (for example, see JP-A-2009-242234). According to the technique, a melting point of a grain boundary phase in alumina particles can be heightened. This ensures reducing softening of the grain boundary phase when the insulator becomes a high temperature.